In general, a catalyst for purifying an exhaust gas is provided in an exhaust passage of an internal combustion engine. When the catalyst is degraded, more harmful substances are emitted into an atmosphere and therefore, for preventing this issue, the degradation of the catalyst is diagnosed. Particularly, in a case of an engine mounted in an automobile, there is also a demand for diagnosing degradation of a catalyst on board from regulations in various countries.
For example, Japanese Patent Application Laid-Open No. H04-060106 (1992) discloses a device in which a temperature increasing grade is calculated based upon a catalyst temperature detected at a steady operating time of an engine and a catalyst temperature detected after an air-fuel ratio is corrected to a rich side at a steady operating time, and it is determined that the catalyst is degraded when the temperature increasing grade is not within an allowable range. In addition, Japanese Patent Application Laid-Open No. H07-180536 (1995) discloses that in a device for diagnosing degradation of a catalyst based upon an output value of a temperature sensor for detecting a temperature in an inlet of the catalyst or in the vicinity thereof and an output value of a temperature sensor for detecting a temperature in an inside or an outlet of the catalyst, the output value of the one temperature sensor is corrected in accordance with an output difference at a predetermined timing between outputs of both the temperature sensors.
The technology described in Japanese Patent Application Laid-Open No. H04-060106 (1992) is as follows. That is, when the air-fuel ratio is corrected to the rich side, a reaction heat of the catalyst increases to increase a temperature of the catalyst. Further, the temperature increasing grade of the catalyst at this time changes with a degradation degree of the catalyst. Accordingly, the degradation of the catalyst is intended to be determined by taking advantage of this characteristic.
However, the temperature increasing degree of the catalyst at the time the air-fuel ratio is changed to the rich side changes not only with the degradation degree but also with a temperature of the exhaust gas flowing into the catalyst. Therefore, without considering this, an accurate catalyst degradation diagnosis cannot be carried out.
Accordingly, the present invention is made in view of the foregoing problems and an object of the present invention is to provide a degradation diagnosis device for a catalyst which can carry out an accurate diagnosis